Corrosion-resistant steel

ABSTRACT

A method of producing a ductile, corrosion resistant steel sheet or foil by depositing a coating of chromium, copper, nickel and/or titanium onto the surface of about an 0.080-inch thick hot rolled pickled steel strip prior to any cold rolling, and then cold rolling the coated steel strip to tinplate or foil gauge. The coated steel may then be annealed at a temperature below the Ac1 temperature to completely recrystallize the steel without causing substantial alloying of the steel and coating metal.

United States Patent [72] Inventor Andrew Lesney Frazer Township, Allegheny County, Pa. [21] Appl. No. 818,813 [22] Filed Apr. 23, 1969 [45] Patented Oct. 26, 1971 [73] Assignee United States Steel Corporation [54] CORROSION-RESISTANT STEEL 5 Claims, N0 Drawings 52 us. C1 148/12, 29/1963, 29/1966, 72/47 [51] Int. Cl ..B32b 15/18, C21d 7/02 [50] Field of Search 148/12; 29/1966, 196.3; 72/47 [56] References Cited UNITED STATES PATENTS 2,731,403 1/1956 Rubin 29/1966 2,753,623 7/1956 Boessenkool et a1. 29/1963 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. W. Stallard Attorney-Forest C. Sexton ABSTRACT: A method of producing a ductile, corrosion resistant steel sheet or foil by depositing a coating of chromium, copper, nickel and/or titanium onto the surface of about an 0.080-inch thick hot rolled pickled steel strip prior to any cold rolling, and then cold rolling the coated steel strip to tinplate or foil gauge, The coated steel may then be annealed at a temperature below the Ac temperature to completely recrystallize the steel without causing substantial alloying of the steel and coating metal.

BACKGROUND OF THE INVENTION This invention relates to the manufacture of corrosion-resistant, coated steels, and more specifically, to an economical method of producing steel sheet or foil having an exceptionally thin protective coating of chromium or a like metal.

In the manufacture of conventional thin gauge, coated steel products, such as tinplate, galvanized sheet, foil and the like, the steel sheet is usually cold rolled to final gauge and annealed prior to application of the protective coating. More recently, processes have been developed wherein a protective coating of chromium or the like is applied to black plate (i.e. uncoated, cold rolled and annealed steel sheet about 0.036- inch thick or less) prior to final cold reduction. That is, the coating is applied after the hot band (i.e. hot rolled steel sheet having a thickness of about 0.080inch) is cold reduced to black plate but before the final cold reduction to final gauge. Although this results in some economic advantages because a lesser amount of coating metal is required, there had been one disadvantage in that the coated steel sheet could not easily be annealed after the final cold reduction without causing the metal coating to alloy with the steel substrate.

U.S. Pat. No. 3,355,265discloses a process whereby black plate, suitably coated with chromium, or a similar protective metal, is cold rolled to fine gauge and subsequently annealed to produce a ductile, corrosion-resistant sheet without causing the metal coating to alloy with the steel. The essential features of that process are that the steel must first be decarburized, if it is not carbon stabilized prior to coating, and after coating and cold rolling, the sheet is annealed in dry hydrogen at below the Ac,temperature for a sufficient time to cause the steel to be recrystallized but insufficient to cause substantial alloying of the coating and steel substrate.

The reason the steel must be decarburized or at least carbon stabilized is because most metals useful as protective coating, such as chromium, will chemically react with uncombined carbon in the steel to form carbides. These carbides are generally very brittle, and when present in sufficient quantities make it impossible to satisfactorily shape the coated steel. Furthermore, the carbides do not offer the same surface appearance or degree of protection to the steel base. The above-cited patent teaches that it is therefore necessary to maintain the active carbon content at a level below 0.007 percent in order to provide a coated steel without undue carbide formation by the coating.

SUMMARY OF THE INVENTION This invention is predicated upon my development of a modification to the above patented process whereby the operation is made less costly in that even a lesser amount of coating metal is utilized. Specifically, I have discovered that if a thin layer of chromium or a similar corrosion-resistant metal is deposited onto a pickled hot rolled sheet, as opposed to the cold rolled black plate, the coated hot rolled sheet can be cold reduced as much as 90percent or more, even by a double cold roll with an intermediate anneal, and still retain a sufficient protective coating as to be a suitable, cheaper substitute for tinplate. In addition, ductility and a bright appearance may be restored by a suitable final anneal. Furthermore, such a thin protective coating does not as readily form carbides, and hence the substrate steel need not be decarburized or carbon stabilized prior to coating.

Accordingly, it is an object of this invention to provide a method of more cheaply producing corrosion resistant sheet steel wherein, prior to any cold reduction, chromium or a like metal is deposited onto a conventional, nondecarburized hot rolled pickled sheet of steel, and then the coated sheet is completely cold rolled to final thickness. If desired, the sheet may be annealed at below the Ac temperature for a time sufficient to substantially recrystallize the steel, but insufiicient to cause substantial alloying between the metal coating and the steel substrate.

It is another object of this invention to provide a method of more cheaply producing ductile, corrosion resistant sheet steel.

It is a further object of this invention to provide a method of producing ductile sheet steel having an exceptionally thin coating of corrosion resistant metal which is effective to render the steel corrosion resistant.

DESCRIPTION OF THE PREFERRED EMBODIMENT According to the present invention, a hot rolled pickled sheet of steel having a conventional thickness of about 0.060 to 0.120 inch is coated with a protective metal deposit, cold rolled to final thickness, and, if desired, annealed to restore ductility. Since the coating is fully cold reduced along with the steel substrate, an exceptionally thin but efi'ective protective coating results. If proper precautions are taken, the coating is not adversely affected by intermediate or final anneals which render the steel substrate ductile.

Coating metals which may be used are those which have substantial corrosion resistance characteristics and a melting point sufficiently high, e.g. above about l,350 F., to permit complete recrystallization of the steel base during annealing without completely alloying with the steel. These metals include chromium, copper, nickel and titanium with chromium being preferred. Although some alloying is desired to insure a chemical bond, i.e. diffusion bond, between the coating and the steel base, substantial alloying of the steel and coating should be avoided in order to maintain the integrity of the coating and the improved properties and the corrosion resistance which it provides. A diffusion bond is much stronger than a mere mechanical bond and is less likely to peel or flake off during cold reduction.

In the practice of this invention l have learned that conventional open hearth steel having carbon contents up to about 0.12 percent do not adversely affect the corrosion characteristics of the coated steel, even after annealing. Although low-carbon hot band steel will be equally acceptable, such low levels of carbon as taught by the prior art practices therefore, the steel substrate of this invention need not be decarburized nor need it contain carbon stabilizers.

Ideally, the corrosion-resistant metal should be deposited onto the hot band to effect a coating thickness of from 0.005 to 0.001 inch, preferably by a vapor deposition technique. Such vapor-deposited coatings are more ductile than those obtained by electroplating, and the process is less costly.

Although, as pointed out above, several different metal coatings are suitable for the practice of this invention, chromium coated products are currently of particular interest to industry. Therefore, specific examples herein will be directed to this embodiment. In a preferred practice of this invention, conventional hot band is used having a low to moderate carbon content, i.e. up to about 0.12 percent. The metal coating, e.g. chromium, may be advantageously applied by exposing a chromium-halogen containing gases to the surfaces of the hot band while the steel is at a temperature of from L650 to 2,100 F. Such chromizing techniques are well known in the industry and need not be further detailed here. For examples, see U.S. Pat. Nos. 2,921,877, 3,059,91 land 3,222,2l2. Once the hot band coil has been coated, it may be cold rolled to any desired gauge according to conventional mill practices. That is to say, except for more carefully and critically annealing parameters, the coated hot band may be completely cold rolled in accordance with conventional cold rolling procedures. Hence, if conventional light or tinplate gauges are desired (i.e. 0.006 to 0.0l49 inch), the coated sheet may be reduced thereto in a single series of cold reductions without an intermediate anneal. As in conventional mill practices, if gauges thinner than about 0.006 inch are desired, it will be necessary to give the sheet a double cold reduction with an intermediate anneal. After the final cold reduction to the desired thickness the sheet may again be annealed to make the sheet ductile.

The object of any intermediate anneal is of course merely to eliminate the work-hardened condition, i.e. recrystallize the steel so that further cold reductions can be easily effected Because of the corrosion-resistant coating on the steel, however, precautions must be taken to prevent the coating from alloying with the steel substrate. Specifically, continuous annealing procedures should be used, annealing temperatures should be kept below the Ac,temperature, and annealing times should be kept at a minimum. Generally, annealing temperatures of about 1,050 to l,300 F. for a period of about 1010 seconds at temperature is most suitable.

A final anneal. like any intermediate anneal, should be at a lcmperaturc below the Ac,temperature and for a time suffi- LlCfli to completely recrystallize the steel but insufficient to cause substantial alloying of the coating and steel. Here again continuous annealing should be utilized, because it is quicker, annealing parameters are less critical, and it is less likely to cause alloying between the coating and the steel.

The final anneal differs from any intermediate anneal in that the final anneal is preferably effected in a high-purity hydrogen atmosphere for the purpose of brightening the coated sheet making it more attractive for commercial use. The final anneal, therefore, should be substantially the same as that described in US. Pat. No. 3,355,265, cited above. As explained in that patent, hydrogen atmospheres of the desired high purity can be obtained by diffusing commercial hydrogen through a heated (450 C.) palladium-silver alloy membrane. Of course, precautions should be taken to maintain the high purity once attained. For example, prior to annealing the system should be carefully purged to avoid contamination.

As stated above, any intermediate anneal and any following final anneal is preferably effected by continuous annealing procedures. This is because every effort must be taken to prevent the coating from alloying with the steel substrate. Therefore, when the coated steel is to be given two anneals, it is essential that both anneals be continuous. However, if only one anneal is to be performed, as where the coated steel is reduced to tinplate gauge, or any reduction process whereby only one anneal is to be effected, than box annealing procedures could be used for the one and only anneal. This is because of only one anneal is performed, this enhanced alloying is usually not sufficient to be seriously detrimental to the corrosion characteristics of the final product. But when two anneals are necessary, the more substantial amount of alloying resulting from box annealing in addition to the alloying resulting from the other anneal, would be too excessive to be tolerable.

If box annealing is to be used in any one-anneal process, the annealing temperatures should be maintained within the range l,050 to l,l50 F. for a period of from lto Shours. In addition, occluded oxidizing gases such as water vapor and oxygen may evolve from the sheet during initial heating. Therefore, it is advisable that the sheet be preheated to about 600 to 900 F and so maintained for a period of about 4hours prior to annealing. After completion, the coated sheet should. be rapidly cooled to a temperature at which no gas-metal reaction will occur, e.g. about 400 F., before exposure to ambient atmospheres. Because box annealing is more prolonged, excessive precautions must be taken to assure noncontamination of the high-purity hydrogen atmosphere.

When one or more continuous annealing procedures are used on the other hand, a slightly higher and broader temperature range of 1,050" to 1.300 F. may be used effectively. Using temperatures at the higher end of the range will permit effective annealing times of less than one minute. Furthermore, because of such short annealing times, the hydrogen atmosphere need not have as high a degree of purity as that required for box annealing. A few parts per million of impurities greater than that obtained by passing the hydrogen gas through a palladium-silver alloy membrane could be tolerated when continuous annealing the coated strip. Hence, commercial high-purity hydrogen is usually satisfactory for continuous annealing.

To illustrate the detailed characteristics of this invention, the following example is typical of the preferred embodiment of this invention. A 0.77-inch thick steel plate having the composition: 0.035 percent C, 0.49 percent Mn, 0.0l0% P, 0.l l% S and 0.035% Si, was hot rolled (0.80 inch) and acid pickled to remove hot-mill scale. The hot rolled steel was then packchromized by placing the steel in a retort and surrounding it with a chromium-bearing compound, i.e. ferrochromium, and there maintained for l2hours at 1,700 F., permitting a chromium rich coating 0.002l-inch thick to be deposited. The surface, at a depth up to 0.00linch, showed 38.5 percent Cr and 0.06% C. The coated steel was then cold rolled to pound base weight (about 0.0099-inch thick) through a fourhigh cold-reducing mill. After cold rolling and annealing, the coating thickness was too variable for a meaningful thickness determination, but X-ray spectrometric analysis revealed the outer 0.00l-inch surface contained 5.0 percent Cr. The cold rolled sample was then given a 4-day weight gain humidity storage test at 85percent humidity at F. The results thereof are shown in table 1 below contrasted to a black plate sample and a tinplate sample.

Table I Results of Humid Storage Test ma a yatlQQifl Weight Gain Pergay gl @llsq. cm.l

Although the coated sheet of this invention was not as resistant to rusting as conventional tinplate, it is apparent from the table that it is far superior in rusting resistance to conventional black plate. As compared to black plate, rust resistance was improved by a factor of about three times. Although tinplate did display superior rust resistance compared to the Crcoated sheet of this invention, it should be remembered tinplate is substantially more costly to produce. Hence for less corrosive applications, the slightly inferior coated sheet of this invention may well be preferable to tinplate due to its substantially lower cost.

It is apparent that many modifications could be made in the above described inventive process without departing from the basic concept. For example, the above-described tinplate gauge steel could be further cold rolled to foil gauge and retain substantially the same corrosion resistance characteristics. in addition it is obvious that other techniques could be employed for applying the protective metal coating onto the steel.

lclaim:

l. A method of producing a corrosion-resistant coated sheet steel, the steps comprising applying a coating of at least one metal from the group consisting of chromium, copper. nickel and titanium onto a pickled hot rolled steel band having a thickness of about 0.060 to 0.12 inch and having a carbon content of up to 0. l 2 percent; cold reducing the coated steel to a thickness of from 0.006 to 0.0149 inch; annealing the cold reduced coated steel at a temperature below the Ac,temperature for a time sufficient to substantially recrystallize said steel, but insufficient to cause substantial alloying of said coating with said steel.

2. A method according to claim 1 in which said coating is vacuum deposited at a thickness of from 0.001 to 0.005 inch.

3. A method according to claim 1 in which the annealing step comprises box annealing at a temperature within the range of about l,050 to about 1,300 F. for a time of from 1 to 5 hours.

less than 0.006 inch and again continuous annealed at a temperature below the Ac temperature for a time sufficient to substantially recrystallize said steel, but insufficient to cause substantial alloying of said coating with said steel.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 5,90 Dated 0ctober26, 1971 Inventor(s) Andre" Lesney It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 40, after "art" should read are not essential in this process. Contrary to prior art Column 3, line &2, after "because", "of" should read if Column 4, line 5, "0.50" should be --o.08o line 63,

"0.12" should read 0.120 line 7 4, "l,300"should read 1,150

Signed and sealed this 7th day of November 1972.

(SEAL) At test:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Atteating Officer Commissioner of Patents RM PO-mSO (10-69} lJSCOMM-DC B0376-P69 n I! S GOVERNMENY PRINTING OFFILE W5; 0" 366*33l 

2. A method according to claim 1 in which said coating is vacuum deposited at a thickness of from 0.001 to 0.005 inch.
 3. A method according to claim 1 in which the annealing step comprises box annealing at a temperature within the range of about 1,050* to about 1,300* F. for a time of from 1 to 5 hours.
 4. A method according to claim 1 in which the annealing step comprises a continuous anneal at a temperature within the range of about 1,050* to 1,300* F. for a period of less than one minute.
 5. A method according to claim 4 in which the cold reduced and annealed steel is further cold reduced to a thickness of less than 0.006 inch and again continuous annealed at a temperature below the Ac1temperature for a time sufficient to substantially recrystallize said steel, but insufficient to cause substantial alloying of said coating with said steel. 